No Supermassive Black Holes in Giant Galaxy Disks: M101 and NGC 6946

The Hobby-Eberly Telescope was used to obtain high-resolution spectroscopy of the nuclear star clusters in the bulgeless, giant Scd galaxies M101 and NGC 6946. Their nuclei have velocity dispersions of 25 to 40 km/s. Any supermassive black holes in these clusters must have masses less than approximately 10**4 to 10**5 solar masses. Similar results are obtained for IC 342 from a published velocity dispersion. These limits are much smaller than masses that are predicted if black holes in bulgeless galaxies correlated with galaxy disk properties such as rotation velocities V in the same way that black holes correlate with elliptical galaxy and bulge properties such as velocity dispersions. Since these are giant galaxies with V = 200 km/s, this result provides an especially stringent check that black holes do not correlate with galaxy disks. All three galaxies contain little or no pseudobulge component, either, a result that can be understood from dynamical arguments. Therefore gas inflow processes like those that occur rapidly in galaxy mergers and slowly in internally driven secular evolution are essentially unavailable for black hole feeding. However, some (pseudo)bulgeless galaxies, including IC 342 and NGC 6946, show weak Seyfert activity, and some are known to contain relatively low-mass black holes. This is a hint that low-mass black holes in bulgeless galaxies and high-mass black holes in bulges and ellipticals may have fundamentally different formation histories.

This work was supported by the National Science Foundation through grant AST-0607490.